Solid Freeform Fabrication of Silicon Nitride Shapes by Selective Laser Reaction Sintering (SLRS)

SelectiveLaser ReactionSinteringis a variation ofselective laser sintering (SLS) that incorporates anjn~situreaction underthe·scannedbeamtofabricate shapes from materials not directly accessible by traditional SLS. Thispaperclescribesaninvestigation into the production of silicon nitride (Si3N4) shapes by lasersinteringsiliconpowderinanammonia (NH3) atmosphere. The effect of gas pressure and the importance of gas/laserinteractionsarediscussed. Single and multiple layer shapes are fabricated. The material is analyzed by x~ray diffraction spectroscopy (XRDS) for phase content and scanning electron microscopy (SEM) for macrostructure. Data is presented that demonstrates conversion rates from silicon to silicon nitride on the order of 85%.Mechanical Engineerin

Development of Nanocomposites for Solid Freeform Fabrication

Nanocomposites in which the constituents are mixed on a nanorneter scale can provide important advantages in the Selective Laser Sintering (SLS) and Selective Laser Reactive Sintering (SLRS) processes. The larger surface area and grain boundaries in the nanocolnposites compared to that in the conventional microcomposites are expected to enhance the solid state diffusion during laser irradiation as well as during any other subsequent processes. Our strategy is to design and develop nanocomposites in which one nanosize cOlnponent has a lower melting point than the other nanosize component, either of which can serve as the matrix phase. The nanoscale dispersion of the low melting component can aid the sintering process during SLS or SLRS. Nanocomposite powders of AI203-COOx, Ab03-NiO, A1203-CO and A1203-Ni have been synthesized by sol-gel processing and are evaluated by SLS.Mechanical Engineerin

Quasinormal modes for tensor and vector type perturbation of Gauss Bonnet black holes using third order WKB approach

We obtain the quasinormal modes for tensor perturbations of Gauss-Bonnet (GB) black holes in $d=5, 7, 8$ dimensions and vector perturbations in $d = 5, 6, 7$ and 8 dimensions using third order WKB formalism. The tensor perturbation for black holes in $d=6$ is not considered because of the fact that it is unstable to tensor mode perturbations. In the case of uncharged GB black hole, for both tensor and vector perturbations, the real part of the QN frequency increases as the Gauss-Bonnet coupling ($\alpha'$) increases. The imaginary part first decreases upto a certain value of $\alpha'$ and then increases with $\alpha'$ for both tensor and vector perturbations. For larger values of $\alpha'$, the QN frequencies for vector perturbation differs slightly from the QN frequencies for tensorial one. It has also been shown that as $\alpha' \to 0$, the quasinormal mode frequency for tensor and vector perturbation of the Schwarzschild black hole can be obtained. We have also calculated the quasinormal spectrum of the charged GB black hole for tensor perturbations. Here we have found that the real oscillation frequency increases, while the imaginary part of the frequency falls with the increase of the charge. We also show that the quasinormal frequencies for scalar field perturbations and the tensor gravitational perturbations do not match as was claimed in the literature. The difference in the result increases if we increase the GB coupling.Comment: 17 pages, 11 figures, change in title and abstract, new equations and results added for QN frequencies for vector perturbations, new referencees adde

Classical Yang-Mills Black hole hair in anti-de Sitter space

The properties of hairy black holes in Einstein–Yang–Mills (EYM) theory are reviewed, focusing on spherically symmetric solutions. In particular, in asymptotically anti-de Sitter space (adS) stable black hole hair is known to exist for frak su(2) EYM. We review recent work in which it is shown that stable hair also exists in frak su(N) EYM for arbitrary N, so that there is no upper limit on how much stable hair a black hole in adS can possess

Solid Freeform Fabrication of Silicon Carbide Shapes by Selective Laser Reaction Sintering (SLRS)

This paper describes an investigation ofthe production ofsilicon carbide shapes by Selective Laser Reaction Sintering (SLRS). One type ofSLRS process, which combines laser sintering of silicon with acetylene decomposition, is briefly outlined, and the mechanisms important to the process are discussed. A series oftest shapes are made at different acetylene pressures to determine pressure effects on conversion to silicon carbide. X-ray diffraction spectroscopy is used for bulk analysis ofthe shapes, and Auger electron spectroscopy is used for surface analysis. The results indicate that acetylene pressure does have a strong effect on silicon conversion to silicon carbide, and SLRS can be used successfully to make silicon carbide shapes.Mechanical Engineerin

Ceramic Joining by Selective Beam Deposition

Current methods ofjoining of ceramic components may compromise the strength, chemical resistance, or high temperature properties of the resulting ceramic parts. A new method of joining, Ceramic Joining by Selective Beam Deposition, creates an all-ceramic joint between two or more ceramic components through selective decomposition of a gas precursor. An all-ceramic joint not only preserves the valuable properties of the ceramic materials joined, but may be tailored to match the coefficient of thermal expansion ofthe original material(s). The added material may be the same as one or both of the joined Inaterials, or may be a composite material. This preliminary work explores the effect of scanning speed and precursor pressure on Selective Beam Deposition ofsilicon carbide using tetramethylsilane.Mechanical Engineerin

Silicon Carbide Shapes By Selected Area Laser Deposition Vapor Infiltration

Selected Area Laser Deposition Vapor Infiltration (SALDVI) is a unique combination ofselected area laser deposition, chemical vapor infiltration and layered powder handling techniques that can be used to fabricate silicon carbide (SiC)/SiC composite shapes. This paper discusses a SALDVI process under investigation which selectively infiltrates SiC powder with SiC generated by decomposition of a gas precursor under a scanned laser beam. A general description of the process, including some of its inherent advantages is presented. Experimental results which explore beam interaction, powder size and infiltration time effects are also presented.Mechanical Engineerin

Selective Area Laser Deposition from Acetylene and Methane to Increase Deposition Control

Selective area laser deposition (SALD) has been used to deposit carbon from acetylene. Working at the relatively high pressures required to produce high deposition rates can result in explosive uncontrollable growth. Pr~vious computational modeling indicates that the energy released from the exothermic decomposition of acetylene to carbon may be responsible for this behaviour[l]. Since methane decomposes endothermically to form carbon over certain temperatures, it is possible that methane addition to the process may help control the deposition rate. The purpose of this paper is to describe SALD experiments that were performed using various partial pressures of acetylene and methane as precursor in order to determine if combining an endothermic and an exothermic reaction effects the control of the SALD process.Mechanical Engineerin

Development of a Selective Laser Reaction Sintering Workstation

The purpose of this paper is to describe the design and operation of a Selective Laser Reaction Sintering workstation developed at The University of Texas. The workstation allows the study of solid freeform fabrication of reaction sintered materials on a research scale. The mechanical and control systems of the workstation are detailed, and Selective Laser Reaction Sintering as a technique is discussed including example material systems that are currently under study.Mechanical Engineerin

Advances in Selective Area Laser Deposition of Silicon Carbide

Selective Area Laser Deposition (SALD) is a Solid Freeform Fabrication (SFF) technique which uses a scanning laser beam to produce solid material by locally decomposing a gas precursor. In this work, a focused C02 laser beam strikes a substrate in the presence oftetramethylsilane (TMS) or diethylsilane (DES), producing silicon carbide objects with high density and no binder phase. Recent investigation has yielded growth rates up to 2.7mlnJmin in the beam area, and has eliminated previously noted contamination ofthe optics by a byproduct which mass spectroscopy identifies as silicon dioxide. This paper reviews a cause of non-uniform growth and delTIOnstrates the addition of hydrogen and reduced scan speeds to lTIake lTIultilayer parts. In addition, it presents a lTIethod for in-situ measurement of height of deposited material.Mechanical Engineerin